Process control plants or systems often employ rotary valves, such as ball valves, butterfly valves, eccentric-disk valves, eccentric-plug valves, etc., to control the flow of process fluids. Rotary valves typically include a fluid flow control member (e.g., a disk, a ball, etc.) disposed in the fluid path and rotatably coupled to the body of the valve via a shaft. To control the flow of fluid through some rotary valves, the position of the fluid flow control member may be varied from a closed position at which the fluid flow control member is in sealing engagement with a seal that surrounds a flow aperture to a fully open or maximum flow rate position at which the fluid flow control member is spaced away from the seal.
Generally, increasing the amount of engagement between the fluid flow control member and the seal when the rotary valve is in the closed position reduces leakage through the valve and increases the amount of breakout torque required to move the fluid flow control member from the closed position toward an open position. Conversely, decreasing the amount of engagement between the fluid flow control member and the seal when the rotary valve is in the closed position tends to increase leakage through the valve and decreases the amount of breakout torque required to move the fluid flow control member from the closed position toward an open position.